1. Field of the Invention
The present invention concerns a method for diffusion-weighted acquisition of magnetic resonance (MR) signals with an image acquisition sequence and an MR system and an electronically readable data storage medium to implement such a method.
2. Description of the Prior Art
In routine clinical practice, diffusion-weighted MR images supply important diagnostic information in stroke and tumor diagnostics. Technically for this purpose, diffusion coding gradients with high amplitude and long duration are combined with a suitable image acquisition module. In order to minimize movement artifacts, a single shot acquisition module is frequently used; echoplanar imaging (EPI) being routinely used. The following parameters are relevant to the quality of the acquired images:
1. signal-to-noise ratio (SNR),
2. geometric deformations,
3. superposition artifacts due to unwanted coherence paths.
In the geometric aspect, static and dynamic deformations occur, both being caused by the high sensitivity of EPI to spatial variations of the basic field amplitude BO. While static deformations are dependent on the basic field magnet and the examination subject (susceptibility), the dynamic deformations (eddy current effects) depend on details of the gradient pulse timing. Particularly when images acquired with different diffusion gradient directions and amplitudes are combined, the dynamic deformations must be kept as small as possible in order to reduce errors in the resulting data (for example anisotropy maps, diffusion maps, tensor data).
Moreover, it must be ensured that only the diffusion-prepared coherence path in the imaging module is acquired. Additionally generated, unwanted coherence paths must be sufficiently strongly suppressed in order to avoid interference artifacts.
Diffusion measurements by means of MR methods have been known for more than 50 years. The most common method for diffusion coding is the monopolar (meaning gradient pulses with only one polarity) spin echo experiment described by Stejskal and Tanner (Journal of Chemical Physics 42 (1965) P. 288-292). This coding scheme has two significant disadvantages:
1. Strong geometric distortions (high proportion of residual exposure fields),
2. Significant loading of the hardware (gradient amplifier (GPA) must provide power for one polarity).
In particular, the strong deformations have motivated the development of a bipolar double spin echo scheme with implicit compensation of eddy current fields (Heid: Proceedings ISMRM 2000, P. 799 and Reese et al.: Magnetic Resonance in Medicine 49: P. 177-182 (2003)). Although the significant disadvantages of the monopolar scheme are eliminated with this, it is at the cost of
1. a longer echo time, and therefore a reduced SNR (approximately 2-5 ms due to the additional RF pulse),
2. interference artifacts due to unwanted signal contributions (FIDs, spin echoes, stimulated echoes) and
3. limited timing parameters due to insolvability of the equations under specific conditions.
The unwanted signal contributions of at least the stimulated echoes can be eliminated by means of suitable dephasing gradients (as described in US 2007/0167732 A1); however
1. the echo time is additionally extended by this (approximately 5-10 ms),
2. interference artifacts due to spin echoes and FIDs remain,
3. the complete elimination of eddy current fields is undermined since the dephasing gradients are not taken into account and
4. the problem remains of the limited parameter range under specific conditions.
By using implicit spoiling, the echo time is reduced, interference artifacts are completely eliminated, and an expanded parameter range is available as described in DE 10 2009 019 895 A1. However, additional compromises must be made in the reduction of eddy current artifacts.